This invention relates to a timed electric switch for supplying current to a load for periods of time which may be made to differ between a first and subsequent operations of the device.
The device finds particular application in the control of heavy direct current applied to window heaters in vehicles, particularly backlite heaters in automobiles and trucks where initial defrosting may require the application of current for an interval of the order of 10 to 15 minutes before switch-off. Subsequently the backlite heater may need to be reactivated for demisting purposes, however the subsequent periods may usefully be less than that of the first. With single period timers employed to date, the interval chosen has had to be a compromise.
It has become increasingly apparent over the last few years that sources of energy are not inexhaustible, that fuels for motor vehicles continue to increase in price, and that all possible savings in operation costs of the vehicle are to advantage. Further, backlite heaters intended for defrosting purposes draw heavy currents, in some instances, of the order of 40 amps, or even more where the trend is to larger glass areas, from a 12-volt car battery supply. At those times when headlights and in-car heaters are also switched on there is heavy competition for the available output from the battery and alternator. If the backlite has merely a simple on-off switch and the heater is used continuously in such conditions, particularly when the car is in stop-and-go traffic, the battery can be run flat.
I have disclosed a backlite timer in may prior Canadian Pat. No. 868,629 issued 13 Apr., 1971 directed to a long interval timing device to which reference may be made for background. The corresponding U.S. patent is U.S. Pat. No. 3,571,665 issued 23 Mar., 1971.
That timer ensures that the heater is not on continuously by providing an interval of operation for defrosting and which can vary to some extent with environmental temperature conditions.
In this present disclosure, an electrical time switching device is described which allows not only an initial long period of operation but also provides the opportunity of having shorter periods of operation for the second and subsequent actuations of the device, such as is beneficial for demisting purposes after initial defrost action.
It is to advantage, and a device is so described, which includes an automatic reset after the automobile has been stopped, so that the next time the backlite heater is required the full initial timing period of operation can be provided. A typical period of operation would initially be 10 minutes with a 5-minute period in each subsequent operation. In some applications, second and subsequent periods of 2.5 minutes will be satisfactory.
To conform with the laws requiring continued improving gasoline consumption efficiency there is also a steady accent in the automobile manufacturing trade on the need to reduce weight. The device here disclosed can replace a switch, pilot-light, wiring harness, connectors, relay and timing circuitry currently employed in timed defrost arrangements, by a single package having typically one-third the weight of the assemblies now employed in the industry. Considerable cost savings per car can flow from lower initial cost and weight and space savings.
As will be further described herein with reference to specific embodiments of the invention, an energy efficient automatic simplified timing device can be constructed with manual actuation and override providing a positive "feel" to the operator and including a pilot-light indicator of essentially infinite life, all in a single package. Prototypes of specific embodiments here described have been delivering currents of 50 amperes both reliably and without any excessive contact heating.